This invention relates to a fluid, method and apparatus for providing buoyancy, particularly for moving heavy objects underwater.
Buoyancy techniques are well known and frequently applied for the movement or retrieval of structures underwater. In general, they comprise a container or bag that is attached to the structure that needs to be moved together with a gas which is used to fill or partially fill the container exerting a buoyant force on the structure allowing it to be lifted.
While this approach is effective in shallow water, it becomes problematic in deeper water. This is because gas being compressible will require to be applied at a pressure exceeding that of hydrostatic pressure in order to provide buoyancy. Furthermore, on moving towards the surface the gas will expand rapidly increasing buoyancy and the rate which the container, together with its tethered structure, rises to the surface accelerates and becomes uncontrollable.
An alternative approach involves the construction of rigid buoyancy elements using syntactic materials which are weighted. These are affixed to the structure and the weights removed by, for instance, a remote operating vehicle from the buoyancy elements. This approach has the disadvantage that once released of their weights, the buoyancy elements exert a sudden upward force which can be difficult to control and could cause damage to subsea equipment, such as ROVs, and personnel.
To tackle this problem, the weight of the structure to be lifted is determined and complex calculations performed so that a suitable amount of buoyancy is provided. However sometimes it is difficult to know the weight of the structure to be lifted and it has been known for calculations to be incorrect, resulting in the dangerous sudden upward movement of the buoyancy elements and attached structure.
Moreover, such buoyancy elements must be returned to the surface when structures of different weights need to be lifted.